The present invention relates to a screw grommet suitable for mounting a member, such as a component or bracket, to a workpiece such as a body panel for an automobile and relates, more specifically, to a screw grommet comprising a shank inserted into a mounting hole in a workpiece and a flange formed at one end of the shank that is larger than the mounting hole in the workpiece, with a cavity formed in the flange and the shank into which a tapping screw can be screwed to join the member to the workpiece.
FIGS. 1 and 2 show a screw grommet 1 of the prior art. The screw grommet 1 is made of plastic, and consists of a shank 5 and a flange 6 formed at one end of the shank 5 that is wider than a mounting hole 3 in a workpiece 2. A cavity 9 is formed in the shank 5 and the flange 6 into which a tapping screw 7 can be screwed. The shank 5 is inserted into the mounting hole 3 in the workpiece 2, bringing the flange 6 into contact with the workpiece. A mounting hole 11 in a member 10, such as a component or bracket, is aligned with the cavity 9, and a tapping screw 7 is screwed into the cavity 9 to join the member 10 to the workpiece 2. This type of screw grommet 1 is intended to attach the member 10 securely even when the workpiece 2 is a thin panel.
In the screw grommet 1 of the prior art shown in FIGS. 1 and 2, rectangular cross-sections of the mounting hole 3 in the workpiece 2 and the shank 5 are intended to prevent the screw grommet 1 from rotating with the turning of the tapping screw with respect to the workpiece 2. However, as is apparent from FIG. 2, with a small mounting hole 3 the grommet is likely to rotate with the turning of a tapping screw when the difference in the length (a) on one side of the mounting hole 3 and the diagonal length (b) of the shank 5 is small. If the tapping screw is not adequately secured in the grommet because of the rotating grommet, the joining force between the member 10 and the workpiece 2 is inadequate.
The screw grommet disclosed in Unexamined Utility Model Application Publication No. 49-25957 has a rectangular cross-section suitable for a rectangular mounting hole in a workpiece. Axial slits are formed in the shank in positions corresponding to the center of the sides of the rectangular cross-section so that the turning tapping screw widens the flange and the shank in a direction perpendicular to the axial direction. However, the slits extend almost the entire axial length of the shank, which reduces the strength of the shank. When a tapping screw is screwed in, there is insufficient resistance to the fastening torque, so the tapping screw cannot be secured properly, and the joining force is not high.
The screw grommet disclosed in Unexamined Utility Model Application Publication No. 55-124618 has engaging sections on the shank adjacent to the flange on the underside of the workpiece, but the shank does not widen when the tapping screw is turned in. As a result, the grommet is likely to rotate with the tapping screw.
The screw grommet disclosed in Unexamined Utility Model Application Publication No. 7-10572 has a shank with a rectangular cross-section and full-length axial slits in the shank corresponding to the corners of the rectangular cross-section. Engaging sections are arranged adjacent to the flange at the middle of respective sides of the rectangular cross-section of the shank. The shank widens when the tapping screw is screwed in, but because the shank in this screw grommet is divided into four sections by slits extending the entire length of the shank, there is insufficient resistance to the fastening torque when the tapping screw is screwed in, and the tapping screw cannot be secured properly.